In recent years, devices having a microstructure formed by micro-machining technology have been applied in various kinds of technological fields. Such devices include, for example, a micro movable device having a microscopic movable part such as a micro-mirror device, angular velocity sensor, and acceleration sensor. The micro-mirror device is used in the field of, for example, optical communication technology or optical disk technology as a device performing an optical reflection function. The angular velocity sensor and acceleration sensor are used, for example, for a vibration preventing function of video cameras and mobile phones with cameras, car navigation system, air bag release timing system, posture control system of cars, robots and the like. Japanese Patent Application Laid-Open Nos. 2003-19530, 2004-341364, and 2006-72252 disclose such micro movable devices.
FIG. 29 to FIG. 32 illustrates a micro movable device X3, which is an example of a conventional micro movable device. FIG. 29 is a plan view of the micro movable device X3. FIG. 30 is a plan view partially illustrating the micro movable device X3. FIG. 31 and FIG. 32 are sectional views along a line XXXI-XXXI and a line XXXII-XXXII in FIG. 29 respectively.
The micro movable device X3 includes a micro movable substrate S5, a wiring substrate S6, a spacer 220, and an adhesive part 230.
The micro movable substrate S5 has a micro movable unit Xb formed thereon. The micro movable unit Xb includes a movable part 201, a frame 202 around the movable part 201, and a pair of connecting parts 203 connecting the movable part 201 and the frame 202. The pair of connecting parts 203 defines an axial center A3 of rotational displacement of the movable part 201. The micro movable substrate S5 has a conduction path (not illustrated) leading to each part such as the movable part 201 and the frame 202 provided therein.
A wiring pattern 210 is formed on the surface of the wiring substrate S6. The wiring pattern 210 includes pad parts 211 and 212. The pad part 211 is an external connection terminal for the micro movable device X3.
As illustrated in FIG. 31, the spacer 220 includes a bump part 221 and a conductive adhesive part 222. The spacer 220 is provided between the frame 202 of the micro movable substrate S5 and the wiring substrate S6. The bump part 221 is pressure-welded to the pad part 212 of the wiring pattern 210 in the wiring substrate S6. The micro movable substrate S5 is bonded to a pad part 202a provided on the surface of the frame 202 via the conductive adhesive part 222. The spacer 220 described above electrically connects the micro movable substrate S5 and the wiring substrate S6. The spacer 220 constitutes a portion of the conduction path from the pad part 211 of the wiring pattern 210 of the wiring substrate S6 to a specific region in the micro movable unit Xb of the micro movable substrate S5.
The adhesive part 230 is an adhesive for fixing the substrates. As illustrated in FIG. 31 and FIG. 32, the adhesive part 230 is provided between the frame 202 of the micro movable substrate S5 and the wiring substrate S6. By providing the adhesive part 230, fixing strength between the micro movable substrate S5 and the wiring substrate S6 is increased.
When the micro movable device X3 is applied to a micro mirror device, a mirror surface 201a is provided on the movable part 201. Further, an actuator (not illustrated) that generates a driving force (electrostatic attraction) to cause a rotational displacement of the movable part 201 around the axial center A3 is provided in the micro movable device X3. The movable part 201 is rotationally displaced around the axial center A3 up to an angle at which electrostatic attraction generated by the actuator and the total torsional resistance of each of the connecting parts 203 are balanced by the actuator being operated. When the driving force of the actuator disappears, the movable part 201 is brought back to the position in a natural state by the restoring force of the connecting parts 203. The reflecting direction of a light signal reflected by the mirror surface 201a provided on the movable part 201 is shifted by such a rocking drive of the movable part 201.
When, on the other hand, the micro movable device X3 is applied to an acceleration sensor, for example, a capacitor electrode for detection (not illustrated) is provided in each of the movable part 201 and the frame 202. The capacitor electrodes for detection are arranged facing each other. In this case, the electrostatic capacity of the capacitor changes in accordance with the rotational displacement around the axial center A3 of the movable part 201. When acceleration acts on the movable part 201, the movable part 201 is rotationally displaced around the axial center A3. Accordingly, the electrostatic capacity between the capacitor electrodes for detection changes. Based on the change in electrostatic capacity, the rotational displacement of the movable part 201 is detected. Based on a detection result of the rotational displacement of the movable part 201, acceleration acting on the micro movable device X3 or the movable part 201 is derived.
In a micro movable device in which a movable part and a frame are connected by connecting parts, a spring constant of the connecting parts affects mechanical characteristics of a device. Thus, in the conventional micro movable device X3, the spring constant of the connecting parts 203 is more likely to vary. Accordingly, mechanical characteristics such as a resonance frequency of the movable part 201 are more likely to vary.
In a manufacturing process of the micro movable device X3, first the bump part 221 is pressure-welded onto the pad part 212 in the wiring substrate S6. Then, a conductive adhesive which later becomes the conductive adhesive part 222 is supplied to a top part of the bump part 221. On the other hand, an adhesive for fixing the substrates to be the adhesive part 230 later is applied onto the wiring substrate S6. After the conductive adhesive and adhesive for fixing the substrates are applied, the micro movable substrate S5 and the wiring substrate S6 are joined via the bump part 221, the adhesive for fixing the substrates, and the like.
When the micro movable substrate S5 and the wiring substrate S6 are joined, the adhesive part 230 is formed by the hardening of the adhesive for fixing the substrates. When hardened, the adhesive for fixing the substrates contracts, for example, in directions illustrated by thick arrows in FIG. 31 and FIG. 32. Thus, the portion of the frame 202 of the micro movable substrate S5 where the adhesive part 230 is bonded is placed under a strong stress. If the frame 202 is deformed due to the stress, a stress also acts on the connecting parts 203. For example, a tensile stress acts on the connecting parts 203 in an arrow D direction illustrated in FIG. 32. Thus, the spring constant of the connecting parts 203 is different before and after the micro movable substrate S5 and the wiring substrate S6 are joined. That is, the spring constant of the connecting parts 203 varies.
The spring constant of the connecting parts 203 also varies due to a temperature change even after the micro movable substrate S5 and the wiring substrate S6 are joined. The adhesive part 230 undergoes a greater change in volume (expansion or contraction) due to a temperature change than, for example, the bump part 221 of the spacer 220. This is because a stress acts on the connecting parts 203 after the frame 202 is deformed due to the change in volume.
In the conventional micro movable device X3, as described above, the spring constant of the connecting parts 203 is more likely to vary. Variations of the spring constant of the connecting parts 203 may cause mechanical characteristics such as the resonance frequency of the movable part 201 to vary. Variations in mechanical characteristics of a device are not desirable because such variations frequently arise as degradation in device performance.